A lithographic projection apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic projection apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. including part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
Often particles, such as hydrocarbon particles or particles of radiation sensitive material, are present in the surroundings of the projection system or released from the substrate, respectively. Particles originating from the substrate may be released either when the substrate, in particular the radiation sensitive material, is exposed to the patterned beam, or when other processing of the substrate occurs, before or after exposure to the patterned beam. Particles, originating from the substrate or present in the surroundings of the projection system, may interact, for example, with an optically active surface, i.e. a surface through which the patterned beam passes and/or changes its direction. Such an optically active surface may also be present in a lithographic apparatus at positions that do not belong to the projection system, such as optically active surfaces that are located towards the end of the projection system, downstream with respect to the propagation direction of the patterned beam. Such an end, hereinafter also referred to as the “bottom” of the projection system, may include, for example, a lens surface or a mirror surface. Such a surface is often a very expensive part of the projection system due to special polishing of such a surface. Hence, the design of the projection system is often such that the dimensions of the optically active surfaces may be kept to a minimum. Interaction of the above described particles with the bottom of the projection system may have a detrimental effect on the intensity and/or accuracy of the patterned beam. A reduction in life-time of the optically active surfaces due to the interaction of the particles with these surfaces, or a removal of a layer of the optically active surface by re-polishing of the surface, may result in a substantial increase in at least the operational costs of the lithographic apparatus.
To reduce the likelihood of these particles reaching a sensitive part of the projection system, i.e. an optically active surface of the projection system, the lithographic apparatus includes a fluid provider that is arranged to provide a flow of purging gas along the optically active surface, across a space between the substrate and the optically active surface of the projection system, or across the substrate. The flow of purging gas protects the optically active surface of the projection system by diverting the particles away from their course towards an optically active surface of the projection system.
In this context it is also worth noting that the current tendency is aimed at more accurate projections that demand a higher numerical aperture number NA. This may be achieved by having the bottom of the projection system close to the substrate. Alternatively, or additionally, it is possible to provide a fluid having a relatively high refractive index, e.g. water, at a space between the bottom of the projection system and the substrate. Such an immersion liquid may also be applied to other spaces in the lithographic apparatus, for example, between the mask and the projection system. Immersion techniques are well known in the art for increasing the numerical aperture of optical systems and projection systems in particular. A lithographic apparatus may be provided with a fluid provider that is arranged to provide the immersion liquid, i.e. a liquid with a relatively high refractive index, or to keep the liquid in its place. The liquid may be flowing to avoid local heating. The fluid provider may thus provide a flow of purging gas, may provide a flow of liquid or may keep a liquid in its place, or may provide a combination of gas and liquid. The application of fluid provider between the substrate and the bottom of the projection system and the limitation of the distance between the bottom of the projection system and the substrate, results in very limited space available for the fluid provider. Consequently, fluid providers are designed so as to meet their functional requirements within a constraint space. Given the above mentioned circumstances it will be appreciated that their is very little space left for replacing the substrate.